Conventional electrical connectors are known for connecting the studs of transformers to wires. A transformer includes an output conductor in the form of a threaded stud which may be connected to a plurality of individual electrical conductors by a transformer stud connector. The most common methods employed for the application of making electrical connections to transformer bushing studs include: (a) screw on, (b) split screw on, (c) slip fit, (d) modified slip fit providing a saddle or nest for the threaded stud, (e) modified slip fit to accommodate two stud sizes, and (f) clamp on. All of these methods can be or have been improved.
The screw on connection relies on a jam nut to maintain a tight interface. Movement of the attached conductors promote slight amounts of torque which cause the screw on bushing stud to loosen, heat up, and eventually fail. Oftentimes, a plurality of conductors is attached to an individual stud. If failure occurs at the electrical interface of the connector or an internal fault in the transformer, all of these conductors must be removed from their respective attachment points to the stud connector. The device is rotated many times to remove it from the stud because it is threaded.
The split screw on connection evolved as a recognition of the loosening of the threaded interface. It provides a split down one side of the threaded connector and a provision for a bolt, or plurality of bolts along this split. When the connector is screwed into place, the bolts are tightened, cinching the connector about the periphery of the stud as opposed to utilizing a jam nut to maintain the secure integrity of the electrical interface. The problem of having to disconnect a plurality of conductors for the purpose of removing the connector is still prevalent.
U.S. Pat. No. 4,214,806 to Kraft discloses a slip fit connection with an internally threaded bore. The inside diameter of the bore is greater than the diameter of the crest of the threaded stud, and having an identical pitch. This connector is slipped over the threaded stud without requiring rotation. Once positioned over the stud, a set screw drives the connector into an eccentric relationship with the stud, causing the threads of equal pitch to nest with one another along the side of the inner bore. This causes a problem with the secure integrity of the electrical interface because the relationship between the stud and the bore of the connector provides only a single line interface.
The fourth type, a modified slip fit device with a saddle or nest for the threaded stud, is disclosed, e.g., in U.S. Pat. No. 5,690,516 to Fillinger. This provides a stepped stud hole having an oversize unthreaded circular hole on top and a slightly smaller intersecting hole on the bottom which provides a mating thread profile and is dimensioned to that of the stud for which it is sized. This structure improves the electrical connection by improving the integrity of the mechanical connection and providing a greater surface area for electrical interface. However, as is well known in the field of mechanical connections of a clamp design, some element of resiliency is required to provide the clamping force. The most prominent example is the elongation of bolt under tensile stress. This tensile stress, when limited within the elastic range of the material, compensates for slight dimensional changes in the bolted joint resulting from thermal changes, maintaining the integrity of the joint.
This resilient clamping force or stored mechanical energy is especially important with electrical connections, since the temperature of electrical connections varies with changes in current. The setscrew or compression screw utilized in the slip fit connectors does not offer the degree of elastic range in the joint as a bolt under tension. These connectors are predominantly aluminum, while the transformer stud bushings are copper. These two materials have differing coefficients of thermal expansion, with the aluminum expanding at a magnitude of approximately 1-½ times the rate of copper for a given increase in temperature. In operation, these connectors typically operate at a thermal rise of as much as 75° C. over ambient. The connector, being aluminum, expands at a rate greater than that of the copper stud. Not having a resilient clamping force, or stored mechanical energy in the connection, the electrical interface becomes loose, resulting in increased resistance to the joint, which results in increased temperature rise.
With the advent of a compound bar design, as taught by the U.S. Design Pat. No. 309,664 to McGrane, a provision is made for two stud receiving bores of different sizes. The two most common thread sizes of transformer bushing studs in the United States are ⅝-11 UNC and 1-14 UNS. Both sizes are in common use, depending on the size of the transformer, and it is advantageous to have a connector which accommodates either size.
The modified slip fit to accommodate two stud sizes is taught by U.S. Pat. No. 6,579,131 to Ashcraft, providing two threaded nests offset from an original slip fit bore similar to the above described modified slip fit. This design illustrates the need for securely mounting a single connector on two different transformer bushing stud sizes, yet the same problem of not providing a resilient clamping force as described above is not provided.
The clamp disclosed in U.S. Pat. No. 6,347,967 to Tamm discloses a stored mechanical energy type electrical connector. This aluminum connector is coupled onto a solid copper stud. The stud has no resiliency to provide to the connection as does a strand conductor. The greater differential of the coefficient of thermal expansion of the aluminum causes such connection to become loose as temperature increases, if it does not have the benefit of stored mechanical energy to offset thermal expansion of the aluminum.
The Tamm electrical connection can accommodate only a single stud size, and therefore, lacks the versatility needed in the present market. Further, the components of this device are not captive, resulting in the propensity of the installer to drop or lose one or more components, particularly the bolt or nut, during installation. The hazards of such loose hardware are readily apparent in an electrical enclosure.
The clamp disclosed in U.S. patent Ser. No. 11/332,479 to Tamm accommodates a transformer bar having streetlight tap wires towards its end opposite the connector component. This presents a difficult configuration for supplementing the transformer bar with streetlight taps and a redundant ground in close proximity to a stud terminal.
Accordingly, a need exists for providing a unique and improved electrical connector for attaching a clamping component to the stud terminal of an electrical device, such as is common on transformer bushings, and for providing an attachment to two different sizes of studs.